Survey data reveal that more than 15% of otherwise healthy older people report having disturbed sleep. The most frequent complaint is an inappropriate timing of sleep, i.e., a very early morning awakening accompanied by profound sleepiness during the evening hours. This phase advance in the circadian rhythms of a number of physiologic and cognitive variables. Animals studies indicate that these age-related advances in the entrained phase of the circadian system are accompanied with a shortening of the intrinsic period of the circadian pacemaker. We have found that there are corresponding age-related changes in the properties of the circadian system in humans that can results in misalignment between the output of the circadian pacemaker and the usual the times available for sleeping and waking. On the basis of our results, four testable hypotheses are proposed: (1) that the intrinsic period of the endogenous circadian pacemaker is shorter in older subjects than in young adults; (2) that the entrained phase of the endogenous circadian rhythm of body temperature is positively correlated with its intrinsic period, and that the primary complaint of early morning awakening is associated with a shorter-than-average period of the endogenous circadian pacemaker; (3) that sleep latency, sleep efficiency, and the timing of REM sleep vary with endogenous circadian phase than is that of younger subjects. An experiment is proposed using new methodologies to allow the accurate and reliable assessment the intrinsic period of the endogenous circadian pacemaker, its relation to entrained circadian phase, and the changes in both of these parameters with advancing age. In this experiment, the influence of age-related changes in the endogenous circadian pacemaker and its impact on sleep structure and circadian phase-dependent sleep disruption will be assessed. This work has significant implications for gerontologic medicine. Sleep disorders in older people are an important cause of a diminished quality of life in otherwise healthy people. They frequently lead to an overconsumption of sleep medications with unfortunate physiological side effects. Greater understanding of the neurophysiologic mechanism underlying sleep disorders in older people as well as understanding of non- invasive methods for their alleviation offer substantive promise for improved health of older people.